Rf in-line impedance matched fuse holder assembly

ABSTRACT

A fuse holder assembly utilizes the distributed capacitance introduced by the placement of a fuse within an interior cavity to compensate for the inductive characteristic of the fuse, thus providing a matched transmission characteristic over a broad frequency range. Since one of the assembly connectors is removably mounted and the connector electrically coupled to the fuse lead by a slidably engaging socket, the fuse is easily replaceable without dismounting the fuse holder assembly from a panel, if it is so employed, by simply removing the connector from the assembly.

i United States Patent Jekai Oct. 28, 1975 RF [bl-LINE IMPEDANCE MATCHED FUSE HOLDER ASSEMBLY Primar i- E.\'aniiner-Harold Broome 75 Inventor: Hans J. Jekai, Redwood City, Calif.

[73] Assi nee: Hewlett-Packard Com an Palo g Am Cam p y 57 ABSTRACT A fuse holder assembly utilizes the distributed capaci- [22] Ffled' July 1974 tance introduced by the placement of a fuse within an [21] Appl. No.: 493,436 interior cavity to compensate for the inductive charac teristic of the fuse, thus providing a matched transmis- [52] U S Cl 337/l91 337/187 sion characteristic over a broad frequency range. [51] H0'lH 85/46 Since one of the assembly connectors is removably [58] Fie'ld 187 188 mounted and the connector electrically coupled to the 5 5 1 fuse lead by a slidably engaging socket, the fuse is easily replaceable without dismounting the fuse holder [56] References Cited assembly from a panel, if it is so employed, by simply UNITED STATES PATENTS removing the connector from the assembly, 2,886,744 5/1959 McN-att, Jr 337/191 1 Claim 2 Drawing Figures I? l l 1 RF IN-LINE IMPEDANCE MATCHED FUSE HOLDER ASSEMBLY BACKGROUND OF THE INVENTION The use of fuses for the protection of radio frequency (RF) circuitry from excessive current has traditionally presented line matching problems. In the earlier instruments, a standard fuse was placed in series with the input transmission line and the impedance mismatch introduced by the inductive characteristic of the fuse went uncompensated. This configuration allowed for the easy replacement of the fuse, but a high voltage standing wave ratio (VSWR) was caused by the inductive electrical characteristics of the fuse. With the advent of circuit miniaturization techniques it became possible to fabricate a transmission strip line on a substrate with a very short, fused interruption. The mismatch created by this configuration also went uncompensated, but since the length of the interruption was much shorter, the inductive impedance was reduced over the above configuration.

Capacitance would normally be used to compensate for the inductive characteristics of the fuse, but in most applications it is not possible to use a discrete capacitor on either end of the fuse. On the input end of the fuse the discrete capacitance may alter the characteristics of the driving or sampled circuit, and a discrete capacitor on the output end of the fuse could affect the response of the front end assembly.

Since compensation through the use of capacitors was not advisable, there has been a continuing effort to minimize the impedance mismatch, and the fused transmission line on a substrate has to date been the best solution. This substrate, however, must be located within the front end housing of the instrument to provide the proper mounting and interconnection sur faces, thus creating serious hazards. The danger arises from the necessity of opening the instrument and its front end housing to replace the fused substrate. The miniaturized circuitry in the instrument, and within the front end particularly, can be very easily damaged by unfamiliar and curious individuals while attempting to replace the fuse substrate. These miniaturized circuits are such that, if they should be damaged it will almost certainly be necessary to replace them, or at least, to perform very costly repairs. This approach also makes the fuse an expensive sole source item which is time consuming to install.

To provide the necessary electrical, as well as packaging protection for the miniaturized circuits, it is desirable to relocate the fuse to a panel position. By so doing, the time and effort to replace the fuse will be greatly reduced, the use of a standard fuse will be possi ble, the excessive current protection will be provided, and capacitance compensation can be attained.

SUMMARY OF THE INVENTION In accordance with the illustrated embodiment, the present invention provides an impedance matched fuse holder assembly which operates over a broad range of frequencies. The impedance matching is necessary since the fuse has inductive electrical characteristics which are substantially matched by the distributed capacitance between the fuse and the cavity which holds the fuse within the fuse holder assembly.

The invention also permits the easy replacement of the fuse since the connectors on the fuse holder assembly are electrically coupled to the fuse leads by means of slidably engaging sockets, and one of the connectors is removably mounted to the fuse holder assembly to permit its easy removal without the necessity of dismounting the fuse holder assembly if it is being employed in a mounted configuration.

DESCRIPTION OF THE DRAWINGS FIG. I is a perspective view of the invention in one of its many embodiments.

FIG. 2 is a cross-sectional view with assembly details of a typical embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Two different embodiments of the invention are represented by FIGS. 1 and 2. FIG. 1 is a representation of a configuration which might be used as a fused transmission cable interconnect on the bench, and FIG. 2 represents a panel mountable configuration. The difference in these two example embodiments is the outer shape of the fuse holder assembly body 19. The exterior shape of the body 19 is not critical to the invention, thus the shape may be varied so that it has a mechanical configuration suitable for the application.

The embodiment represented by FIG. 2 further depicts a cross-section of a typical assembly of the invention. The assembly includes the fuse holder assembly body 19 of an electrically conductive material with an interior cylindrically-shaped cavity 29, with one connector receptacle at each end of the above-mentioned cavity; a cylindrical fuse 15 with an attached coaxial lead protruding from each end mounted within the cavity 29; an insulating means 17 which encircles the body of and runs along the length of the fuse 15 thus supporting the fuse; and a first and a second RF connector, 11 and 13, each mounted in an above-mentioned receptacle on the fuse holder body 19, and each electrically coupled to a slidably engaging socket 21 for receiving the fuse 15 leads therein. It should be noted that the fuse leads may be attached to the ends of the fuse by any convenient technique that provides good electrical contact, such as, soldering or welding by either the manufacturer or the user of the fuse. This type of fuse is marketed by Littlefuse and is marketed as a Pico Fuse Model 275. In addition, at least one of the connector receptacles is machined, such that the connector is removably mounted to permit replacement of the fuse 15. The machining of the connector receptacle might consist of threads of a particular configuration within the receptacle, or of other smaller threaded receptacles in close correspondence to the larger connector receptacle. The RF connectors 11 and 13 in FIG. 2 represent BNC and SMC types, respectively, but the invention is not limited to the use of these types of connectors. During the fabrication process of the fuse holder assembly body 19, provision can be made for the use of any desired type of RF connector such that proper mounting of the connector and electrical coupling of the connector to the fuse 15 can be achieved.

The assembly procedure for the fuse holder assembly depicted by FIG. 2 is to first couple a slidably engaging socket 21 (similar to an IC socket) to both connectors 1 l and 13, to provide the necessary electrical coupling to the leads of the fuse 15. The slidably engaging socket would be soldered to the lug on the BNC connector 11, and this solder joint would be supported within the connector receptacle by the non-conductive bushing 23 behind and attached to connector 11. This support is necessary to provide strain relief for the electrical coupling point when the BNC connector 11 is attached to the fuse holder assembly body 19. Connector 13, an SMC type, at the opposite end of the fuse holder assembly body 19, would be modified by cutting the solder lug close to the pressure fit ring, then inserting the slidably engaging socket 21 into the SMC connector to make electrical contact, and finally filling the cavity of the connector with epoxy to support this connection as does the non-conductive bushing 23 for connector 11. The final step of assembly is to place the fuse 15 within its insulating means 17 into the cavity 29 and then mounting the connectors l 1 and 13 on the fuse holder assembly body 19 in the connector receptacles. The fuse holder assembly of FIG. 2 is then ready for panel mounting using the body threads 25 and the mounting shoulder 27, or as a stand-alone, fused, transmissionline interconnection device on the bench. in either application, the fuse 15 can be changed by dismounting the removably mounted connector 11 or 13. Thus, when the fuse holder assembly is panel mounted, the fuse 15 can be changed without the necessity of opening the instrument. The invention provides easy access to the fuse 15, allows its rapid replacement with a standard fuse, and prevents inadvertent damage to the delicate circuitry within the instrument since it is not necessary to open the instrument.

Unlike the prior art, this invention provides capacitive impedance matching of the inductive impedance characteristic of the fuse 15 over a broad range of frequencies. The capacitance is the result of the placement of the fuse 15 within the cavity 29 in close proximity with the conductive material of the fuse holder assembly body 19. The fuse insulating means 17 which supports the fuse 15 within the cavity 29 provides the dielectric for the distributed capacitance between the fuse l and the cavity 29. By the proper choice of cavity 29 size and dielectric material (insulating means 17) for a given area of conduction within the fuse, the capacitive impedance which is distributed over the body of the fuse can be made substantially equal to the inductive impedance of the fuse 15, thus achieving substantial impedance match. The bushing 23 within the receptacle for the BNC connector 11 provides dielectric for distributed capacitance between the first slidably engaging socket 21 and the surface of the receptacle for the first connector 11. Substantial impedance matching is achieved in this section of the transmission path by the proper choice of the dielectric material of the bushing 23. Since the match is achieved through the use of distributed capacitance, it is achieved incrementally and thus only a very small capacitive load is presented to the driving circuit on the fuse 15 input, or the driven circuit on the fuse 15 output, with little or no effect on the operation of either. Tests of the invention indicate that the match (low VSWR) is very good up to 1.5 61-12, and is acceptable to 2.0 GHZ.

1 claim:

1. A fuse holder assembly incorporating impedance matching over a broad frequency range, said assembly comprising:

a first connector and a second connector;

a body of electrically conductive material having a cylindrically-shaped cavity therein and receptacles at opposite ends of the cavity for mounting thereto the first and second connectors;

a cylindrical fuse disposed within the cavity, the fuse having an inductive electrical characteristic and having a coaxial lead attached in good electrical contact therewith at each end thereof;

insulating means with selected dielectric characteristics disposed intermediate the fuse and the surface of the cavity within said body to support the fuse within the cavity and to provide distributed capacitive reactance between the fuse and the surface of the cavity which is substantially equal to the inductive reactance of the fuse over a broad range of operating frequencies; slidably engaging socket attached to each of said first and second connectors for receiving the fuse leads therein, the sockets having an inductive electrical characteristic and the sockets being in good electrical contact with the connectors for transmitting a signal through the fuse between the first and second connectors; threaded mounting means disposed on at least one receptacle in the body for removably mounting at least one of the first and second connectors to permit the replacement of the fuse; and

a non-conductive bushing with selected dielectric characteristics disposed within the connector receptacle in supporting relationship to a connector which electrically couples to the slidable engaging socket external to the body of said connector for supporting the connector-socket coupling point and for providing distributed capacitive reactance between the slidably engaging socket and the surface of the connector receptacle which is substantially equal to the inductive reactance of the slidably enaging socket over a broad range of operating frequencies.

UNITED STATES PATENT AND TRADEMARK OFFICE CERTIFICATE OF CORRECTION PATENT NO. ,3 3

DATED October 28, 1975 |NVENT0R(5) 3 Hans J. Jekat It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In the title page, column 1, the Inventor's name should read Hans J. Jekat Column 4, line 50, "enaging" should read engaging Signed and Scaled this twentieth D of n y 9 6 [SEAL] Arrest:

RUTH C. MASON C. MARSHALL DANN Attesling Officer (mnmissr'mwr vj'Parents and Trademarks 

1. A fuse holder assembly incorporating impedance matching over a broad frequency range, said assembly comprising: a first connector and a second connector; a body of electrically conductive material having a cylindrically-shaped cavity therein and receptacles at opposite ends of the cavity for mounting thereto the first and second connectors; a cylindrical fuse disposed within the cavity, the fuse having an inductive electrical characteristic and having a coaxial lead attached in good electrical contact therewith at each end thereof; insulating means with selected dielectric characteristics disposed intermediate the fuse and the surface of the cavity within said body to support the fuse within the cavity and to provide distributed capacitive reactance between the fuse and the surface of the cavity which is substantially equal to the inductive reactance of the fuse over a broad range of operating frequencies; a slidably engaging socket attached to each of said first and second connectors for receiving the fuse leads therein, the sockets having an inductive electrical characteristic and the sockets being in good electrical contact with the Connectors for transmitting a signal through the fuse between the first and second connectors; threaded mounting means disposed on at least one receptacle in the body for removably mounting at least one of the first and second connectors to permit the replacement of the fuse; and a non-conductive bushing with selected dielectric characteristics disposed within the connector receptacle in supporting relationship to a connector which electrically couples to the slidable engaging socket external to the body of said connector for supporting the connector-socket coupling point and for providing distributed capacitive reactance between the slidably engaging socket and the surface of the connector receptacle which is substantially equal to the inductive reactance of the slidably enaging socket over a broad range of operating frequencies. 